Needs exist for the sensitive measurement of small angles of roll between separated objects, i.e., of relative rotation about an axis in space connecting the two objects. An electro-optical roll angle detector filling such needs lends itself to combinations that involve known autocollimator principles, but afford simultaneous measurement of relative angles of roll, pitch, and yaw between separated objects or bodies. Such combinations are useful, e.g., in measuring the alignment of bored holes; the roll detector is useful as a pickoff for the stable platform in guidance and navigation systems.